Fix opentracing and Prometheus metrics for replication requests (#10996)

This commit fixes two bugs to do with decorators not instrumenting
`ReplicationEndpoint`'s `send_request` correctly. There are two
decorators on `send_request`: Prometheus' `Gauge.track_inprogress()`
and Synapse's `opentracing.trace`.

`Gauge.track_inprogress()` does not have any support for async
functions when used as a decorator. Since async functions behave like
regular functions that return coroutines, only the creation of the
coroutine was covered by the metric and none of the actual body of
`send_request`.

`Gauge.track_inprogress()` returns a regular, non-async function
wrapping `send_request`, which is the source of the next bug.
The `opentracing.trace` decorator would normally handle async functions
correctly, but since the wrapped `send_request` is a non-async function,
the decorator ends up suffering from the same issue as
`Gauge.track_inprogress()`: the opentracing span only measures the
creation of the coroutine and none of the actual function body.

Using `Gauge.track_inprogress()` as a context manager instead of a
decorator resolves both bugs.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/10996.misc b/changelog.d/10996.misc
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index 0000000000..c830d7ec2c
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+++ b/changelog.d/10996.misc
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+Fix a bug introduced in Synapse 1.21.0 that causes opentracing and Prometheus metrics for replication requests to be measured incorrectly.